Bake in the preheated oven until almonds are golden brown and fragrant, 15 to 20 minutes. The nuts will continue to cook after removing from oven. Transfer hot almonds to a wide-bottomed glass or stainless steel bowl.
Heat olive oil, garlic, rosemary leaves, red pepper flakes, and sea salt in a saucepan over low heat, just before almonds are out of the oven. Mash the garlic and rosemary in the hot oil to release flavor. Pour hot oil mixture over almonds; stir almonds every 5 to 10 minutes until completely cooled.
Drain oil from the almonds; transfer almonds to a paper towel-lined plate. The almonds will still be shiny and oily to the touch.